Behaviour of concrete beams reinforced with different GFRP-steel combinations without and with fire exposure: experiments and analysis

The current research is mainly focused on the use of glass fiber reinforced polymer (GFRP) bars as internal reinforcement for simply supported concrete beams and the use of steel reinforced polymer (SRP) system externally for debonding and confinement investigation. Since GFRP and SRP are newly developed strengthening materials, a better understanding of the mechanical behaviour of these composite systems is needed. The objective of this research program is to investigate a number of issues related to GFRP bars, and SRP composite strips: 1) To investigate the shear strength of concrete beams reinforced with GFRP bars. 2) To investigate the debonding mechanism between SRP composites strips and the concrete surface, using two different set-up; concrete specimens were tested using the single-lap shear test set-up and specimens were tested using a three-point bending set-up, and compare the results obtained based on two different test set-ups. 3) To investigate the behavior of short concrete prisms with a square cross-section confined by SRP composite strips. A total of 96 concrete (beams, prisms) were tested in the course of this research; 14 as part of a shear strength of concrete beam reinforced with GFRP bar study, 7 as part of the debonding phenomenon study, 19 as part of the bond study, 31 as part of the fracture mechanics study and 25 as part of the confinement study.

Concrete beams are commonly used in construction projects to provide structural support. Reinforcing these beams with steel and Glass Fiber Reinforced Polymer (GFRP) can increase their strength and durability. Steel reinforcement is a traditional method used for decades, while GFRP is a newer material that offers several advantages, including corrosion strength and lighter weight. Combining both materials to reinforce concrete beams can result in a stronger and more resilient structure, making it an ideal choice for many construction projects. In this study, the behavior of reinforced concrete beams with steel and GFRP reinforcement is numerically investigated, and the effect of steel percentage and GFRP percentage on the mechanical strength and energy response of the beam is determined using the Response Surface Methodology (RSM). Using the ABAQUS software, a widely used Finite-Element Analysis (FEA), the numerical modeling is first verified, and then targeted analyses are performed on the beam using the tests specified by the RSM. Then, based on the strength and energy of the beam, a two-variable Analysis of Variance (ANOVA) and two-objective optimization are performed to investigate the effect of changing each parameter of steel percentage and GFRP percentage on beam strength and energy. Several laboratory studies have been conducted on the behavior of concrete beams with steel and GFRP bars, but no research has statistically and numerically examined their behavior. We also simultaneously optimized both strength and energy parameters based on the ratio of steel and GFRP and compared them with numerical values. We show an interaction relationship between steel and GFRP ratio in the strength and energy of concrete beams. In the beam with hybrid reinforcement, with the increase of steel and GFRP ratios, the amount of strength and energy does not increase linearly, and there is a quadratic relationship between them.

This paper aims to experimentally investigate the flexural behavior of concrete beams reinforced with glass fiber reinforced polymer (GFRP) bars made of concrete with and without polyethylene terephthalate (PET) waste fiber addition. The test variable covers the structural application of different PET fiber lengths (20 mm and 40 mm). Firstly, several tests were done on ten concrete mixes containing 1–2% PET waste fiber with a 0.25% increment by volume. The mixture's concrete properties, including stress-strain curves, were studied to obtain the optimum percentage of PET fiber. Furthermore, four GFRP reinforced concrete (RC) beams were cast from concrete containing 0% PET fiber (the control beam), 1% PET fiber volumes of 20 mm and 40 mm, and an equal mixture of both lengths. The beams were designed using ACI 440.1R to fail in GFRP rupture. The results showed that compressive strength increased by 8% and 6% using 1% fiber volume of 20 and 40 mm PET fiber, respectively. It was found that PET fiber addition had a negligible effect on changing the ductility, cracking pattern, or failure mode of under reinforced GFRP RC beams. The cracking and ultimate loads are slightly reduced, however, and the corresponding deflections are well reduced, especially when using 40 mm PET fiber. With the use of PET waste fiber, serviceability increased by up to 12% based on deflection limit control (L/240). The addition of 40 mm of PET fiber was found to enhance the beam stiffness by 25%. An analysis method has been proposed to predict the flexural strength of GFRP RC beams made with PET fiber. Moreover, it is recommended to treat the PET fiber surface to improve the fiber-concrete bond when used in non-ductile RC beams.

The bond performance of glass fibre reinforced polymer (GFRP) bars and that of steel bars embedded in Alkali Activated Cement (AAC) concrete are analysed and compared using pull-out specimens. The bond failure modes, the average bond strength and the free end bond stress-slip curves are used for comparison. Tepfer’s concrete ring model is used to further analyse the splitting failure in ribbed steel bar and GFRP bar specimens. The angle the bond forces make with the bar axis was calculated and used for comparing bond behaviour of ribbed steel bar and GFRP bars in AAC concrete. The results showed that bond failure mode plays a significant role in the comparison of the average bond stress of the specimens at failure. In case of pull-out failure mode, specimens with ribbed steel bars showed a higher bond strength while specimens with GFRP bars showed a higher bond stress in case of splitting failure mode. Comparison of the bond stress-slip curves of ribbed steel bars and GFRP bars depicted that the constant bond stress region at the peak is much smaller in case of GFRP bars than ribbed steel bars indicating a basic bond mechanism difference in GFRP and ribbed steel bars.

This study focuses on using various machine learning (ML) models to evaluate the shear behaviors of ultra-high-performance concrete (UHPC) beams reinforced with glass fiber-reinforced polymer (GFRP) bars. The main objective of the study is to predict the shear strength of UHPC beams reinforced with GFRP bars using ML models. We use four different ML models: support vector machine (SVM), artificial neural network (ANN), random forest (R.F.), and extreme gradient boosting (XGBoost). The experimental database used in the study is acquired from various literature sources and comprises 54 test observations with 11 input features. These input features are likely parameters related to the composition, geometry, and properties of the UHPC beams and GFRP bars. To ensure the ML models' generalizability and scalability, random search methods are utilized to tune the hyperparameters of the algorithms. This tuning process helps improve the performance of the models when predicting the shear strength. The study uses the ACI318M-14 and Eurocode 2 standard building codes to predict the shear capacity behavior of GFRP bars-reinforced UHPC I-shaped beams. The ML models' predictions are compared to the results obtained from these building code standards. According to the findings, the XGBoost model demonstrates the highest predictive test performance among the investigated ML models. The study employs the SHAP (SHapley Additive exPlanations) analysis to assess the significance of each input parameter in the ML models' predictive capabilities. A Taylor diagram is used to statistically compare the accuracy of the ML models. This study concludes that ML models, particularly XGBoost, can effectively predict the shear capacity behavior of GFRP bars-reinforced UHPC I-shaped beams.

This paper presents experimental results on the influence of the spiral anchor system on the flexural behavior of concrete beams reinforced with glass fiber-reinforced plastic (GFRP) bars. The experimental program consisted of eight beams with the spiral anchor system and two control fiber-reinforced concrete beams without any spiral anchor system. All specimens were tested under bending load. Rough and smooth surface textures of GFRP bars were considered. The test parameters were the diameter of spiral anchor and the condition of the GFRP reinforcement bars as either bonded or unbonded to the surrounding grout. The experimental results indicate that beams reinforced with a rough GFRP bar with an anchor system under flexural load had higher ultimate flexural strength, first crack strength, and stiffness as compared to the beams without an end anchor system. The success of the anchor system is attributed to the confining effect of the steel spiral in anchoring the reinforcement ends. This confining effect enhances the anchorage capacity of the anchor system and subsequently improves the overall flexural performance of the reinforced concrete beams.

abstract: There is a recurring need to construct in places where environmental aggressiveness is very high, such as tidal-splash sites, chemical industries, etc. In these places, steel bars, commonly used for concrete reinforcement, can suffer deterioration, losing cross-sectional area and consequently the resistant capacity. In this regard, Glass Fiber Reinforced Polymers (GFRP) bars can replace steel because of its high strength to harsh environments, low weight and high tensile strength. Thus, this work aimed to compare reinforced concrete beams with steel bars and GFRP bending bars using the procedures indicated in ABNT:NBR 6118 and ACI 440.1R-15, respectively. Experimental three-point flexural tests were performed on six concrete beams, three reinforced with steel bars and three reinforced with GFRP bars. The beams were designed for centered point loads of 23.5 kN, 37.5 kN and 57 kN, and for each load one beam was reinforced in steel and one in GFRP. As main conclusions, it can be said that the beams reinforced with GFRP bars presented greater transverse displacements due to the low modulus of elasticity of this material. In addition, the beams presented rupture loads close to each design load, showing agreement in the recommendations of the two normative documents. Comparing the maximum loads of steel and GFRP beams, ratios of +9.3%, -3.2% and -3% were obtained for beams designed for 23.5 kN, 37.5 kN and 57 kN, respectively. Also, that variations in design loads cause greater variation in the longitudinal reinforcement rate of GFRP bar-beams compared to steel-bar beams.

Corrosion of steel reinforcement has been identified as a key factor of deterioration and structural deficiency (Masoudi et al., 2011) in reinforced concrete (RC) structural members. The corrosion state of current RC bridges and high-rise buildings has been a source of concern to designers and engineers. In addition, such structures have been invulnerable to harsh environmental exposures, with little or no maintenance. Furthermore, such structures are experiencing larger amount of loads than their original capacities due to the increase number of users over the years (Bisby, 2003). Several different solutions were proposed to retrofit deteriorated structural members (Masoudi et al., 2011; Hawelih et al., 2011; AlTamimi et al., 2011) by replacing cracked concrete, using epoxy injected supplements, and FRP externally bonded systems. The use of embedded FRP bar reinforcement seems to be a promising solution (Masoudi et al., 2011; Bisby, 2003; Abbasi & Hogg, 2005; Abbasi & Hogg, 2006; Qu et al., 2009; Aiello & Ombres, 2002) to strengthen structural RC members in flexure and shear. Compared to the conventional reinforcing steel bars, the FRP bars seem to have a high strength to weight ratio, moderate modulus of elasticity and resistance to chemical and electrical corrosion. Although FRP materials were shown to have a brittle failure, due to their natural composition, still if designed properly they can show considerable amount of ductility (Rasheed et al, 2010; De Lorenzis & Teng, 2007). One of the draw backs of using FRP embedded bars is their low glass temperature and tendency to change state; from solid to liquid at elevated temperatures. Hence, the performance of FRP reinforced structural members under elevated temperatures draws many doubts and concerns and warrants further investigation. Few experimental tests have been conducted in the previous years on the fire performance of RC beams reinforced with FRP bars due to the high costs of such tests, tremendous amount of preparation, and shortage of specialized facilities (Franssen et al., 2009). Sadek et al. (Sadek et al., 2006) conducted a full scale experimental program on the fire resistance of RC beams reinforced with steel and Glass Fibre Reinforced Polymer (GFRP) bars. The test matrix composed of different reinforcing rebars used along with different concrete compressive strengths. The testing took place in a special testing facility and the beams were loaded statically at 60% of their ultimate load capacity during the course of the fire test. The tests followed the ASTM E119 (ASTM E119, 2002) standard and fire curve.

Reinforced concrete structures play an increasing important role in civil and industrial construction, along with the technology of new concrete materials and their calculation theories. In order to improve the crack resistance of three-layer reinforced concrete (3L-RC) beams with the middle layer of lightweight concrete (LWC) material, the method of using polymer bars in place of steel bars is applied. In this study, the behavior of 3L-RC beams with steel bars and glass fiber reinforced polymer (GFRP) bars is simulated and analyzed by ANSYS software (base on the finite element analysis). The simulation of reinforced concrete (RC) beam work on computer software is a modern method, which allows to input many mechanical, physical properties of materials and geometric parameters of 3L-RC beams. The results of the samples beam analysis showed that the cracking resistance of 3L-RC beams with GFRP bars have been enhanced more than twice compared to 3L-RC beams with steel bars. Numerical modeling allows comparison between the obtained results and building theoretical dependences in a wide range of specified parameters in sectional construction of multilayer reinforced concrete elements. This would limit the number of actual test samples, increase the efficiency of experiments.

Research progress on short-term mechanical properties of FRP bars and FRP-reinforced concrete beams

Glass fiber-reinforced polymer (GFRP) bars are used as internal reinforcement in many structural applications. The structural performance of GFRP-reinforced concrete elements is dependent on the physical and mechanical properties of GFRP reinforcement. There is a lack of experimental data on the flexural behavior of concrete beams reinforced with ribbed GFRP bars. This study evaluates the flexural strength and serviceability performance of concrete beams reinforced with ribbed GFRP bars. A total of 11 GFRP-reinforced concrete beams with dimensions of 4,350 × 400 × 200 mm (length × height × width) were constructed and tested under a four-point loading test setup. The main test parameters were the concrete cover, reinforcement ratio, bar spacing, and confinement due to the transverse reinforcement in the bending zone. The results uantify the effect of increasing the reinforcement ratio on the increase in the ultimate capacity and the reduction in deflection at the service and ultimate stages. In addition, the results showed that the increase in the confinement in the bending zone due to closely spaced stirrups resulted in a higher ductility index and ultimate capacity with no considerable effect on the postcracking stiffness of the beams. Moreover, based on the experimental results, the accuracy of deflection equations available in design codes and guidelines is evaluated and discussed.

This study exhibits the bending performance of reinforced concrete (RC) beams with glass fiber-reinforced polymer (GFRP) rods. The beams were strengthened with the bottom or side near surface mounted (BNSM or SNSM) methods. Applying BNSM strengthening to the RC beam is difficult in some cases, so the SNSM became essential. Due to the limited research dealing with this type of beam, this study focuses on the effect of internal and NSM reinforcement areas and the NSM position on the response of the beams. Two un-strengthened and six strengthened RC beams were tested in bending. The GFRP RC beams upgraded with SNSM GFRP bars showed higher deflection, energy dissipation, ductility, and ductile failure than those upgraded with BNSM ones. On the contrary, the load-carrying capability and flexural stiffness of GFRP RC beams strengthened with BNSM GFRP rods were slightly higher than those strengthened with SNSM GFRP. Consequently, a parametric investigation relying on the verified finite element (FE) model was executed. This parametric work aimed to add additional data to the literature related to the influences of the internal and NSM reinforcement type and area on the performance of beams with ‎SNSM. According to the FE results, increasing the NSM reinforcement area improved the capacity and stiffness of the upgraded beams regardless of the combination of the internal and NSM reinforcement. The relative axial stiffness between internal and NSM materials significantly impacted the beam performance. Moreover, using GFRP with CFRP or steel with CFRP indicated higher performance than using steel with GFRP, regardless of which one was located as NSM.

This paper evaluates the shear capacity of concrete beams longitudinally reinforced with glass fiber-reinforced polymer (GFRP) bars. All beams were provided with steel stirrups as shear reinforcement within shear region with spacing, 150 mm. The effect of longitudinal reinforcement ratios, ρ f is discussed and compared with conventional beams reinforced with steel. The test results were compared with predictions recommended by ACI 440.1R-06 design guidelines and by other references. Other performance in terms of tensile strength and stresses in reinforcement bars was analyzed. Based on the findings of test results, it is shown that the shear resistance of GFRP RC beam is influenced by modulus elasticity of GFRP bars that resulted in low shear strength compared to conventional beams. Moreover, due to specific properties of GFRP materials, shear behavior of beams with GFRP bars is different to that of beams with steel. The current design guideline (ACI 440.1R-06) was found to be very conservative compared with experimental results.

<p>This paper reports the experimental and numerical studies of concrete beams reinforced with glass fibre-reinforced polymer (GFRP) reinforcing bars with and without the addition of steel fibres. GFRP- reinforced concrete beam specimens of equivalent geometry were produced and tested under symmetrical two-point loading configuration. Deformation and cracking behaviour were monitored during the test, and the curvature was determined from the measured deformation response over the pure bending zone. In view of the lower stiffness of GFRP bars compared to conventional steel bars, the effectiveness of adding steel fibres to increase the flexural stiffness is investigated. Experimental results show that the steel fibres could reduce the average crack width and deflections of the beam, and could lead to a more ductile failure mode. The beam specimen was numerically analysed by employing the nonlinear finite element programme ATENA, and the analytical results are in good agreement with the experimental results.</p>

The strengthening of reinforced concrete structures using advanced fiber reinforced polymer (FRP) composites is a very popular practice because they are light and highly resistant to corrosion. In particular, the behavior of FRP-reinforced concrete structures is a topic that attracted lot of interest in recent years. However, the application of FRP-reinforced concrete structures requires an extensive development of reliable design equations.This study focuses on the behavior of beams reinforced with different reinforcement ratios of glass fiber reinforced polymer (GFRP) bars and concrete strength. Displacement, strains, and crack width were measured to study the behavior of beams. The results of the investigation can be summarized as follows: (1) Deflections and strains of concrete beams reinforced with GFRP re-bars are generally larger than those reinforced with steel bars; (2) the strength of the concrete has a negligible effect on crack spacing and crack width; (3) and the FRP over-reinforced concrete beams in this study are safe for design in terms of deformability.